Extractive distillation of alcohols from close-boiling aqueous mixtures



May 8, 1951 E. R. GILLILAND E-rAL 2,551,593

EXTRACTIVE DISTILLATION 0F ALCOHOLS FROM CLOSE-BOILING AQUEOUS MIXTURES 2 Sheets-Sheet 1 Filed Jan. 28, 1947 lum-:200mm mm. *Y O m dm Zuumd 0@ 1 0N PN dmmzmzou ----,f w Il mw duw F|||l AI Amd. [lll y N (Aw 2 o -IL l@ Illv .$.23 dmmum lv .uuZmumm L s@ t a |...wv lu dmmimnz u VI. .NW m

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Clttorrze' May 8 1951 R GILLILAND ETAL 2,551,593

E. EXTRACTIVE DISTILLATION 0F ALCOHOLS FROM CLOSE-BOILING AQUEOUS MIXTURES 2 Sheets-Sheet 2 Filed Jan. 28, 1947 Edwin '2, Gilliland Charles E. morrell Cdr'z carzon not C, Qobertson 5158rlbors `points and water miscibilities.

Patented May 8, 1951 UNITED STATES `PATENT OFFICE EXTRACTIVE DISTILLATION OF ALCOHOLS FROM CLOSE-BOILING AQUEOUS MIX- EURE S ration of Delaware Application January 28, 1947, Serial No. 724,840

(Cl. 20E-39.5)

13 Claims.

This invention relates to a practical method of separating close-boiling oxygenated organic compounds, particularly to isolate water-miscible alcohols of different molecular weights which form close-boiling aqueous azeotropes, such as ethyl and isopropyl alcohols. It is concerned with a controlled use of water as a relluxing medium in a continuous fractional distillation of the closeboiling alcohols.

.It is known that water has been used in steam and azeotropic distillations of alcohols and other organic compounds of widely divergent boiling For example, in the'rectication of ethyl alcohol from an aqueous mixture containing amyl alcohol (fusel oil), the

aqueous Iazeotrope of ethyl alcohol is distilled overhead while the amyl alcohol, which has a `considerably higher boiling point than ethyl alcohol andis less miscible with water, concentrates in the middle of 'the fractionatng column,`1eav `ing the .remaining water at the bottom of the column. However, in a normal distillation .of

vclose-boiling alcohols from their aqueous mixtures, these alcohols concentrate in the upper part of a fractionating column with azeotropic proportions of water and the mixed close-'boiling alcohols distill overhead with a limited amount of water. Accordingly, it has been dilcult to separate the alcohols which boil .closely together, especially those of similar .water miscibilties and which form aqueous azeotropes that boil closely together. Such close-boiling alcohols are frequently obtained in aqueous mixtures with varying amounts of water in their syntheses. Since these compounds are generally similar in miscibilities with various solvents and are accompanied with varying proportions of water and organic impurities, extraction methodsarediicult to use for their separation, also.

An objectof this inventionis toprovide a commercially feasibleprocess for eflicient separation of close-boiling alcohols which are similarly miscible with Water and which are difficult to separate rby ordinary fractional distillation methods.

The process with which the present invention is concernedis best applied to distillation cuts or mixtures-ofwhich the organic components distill as compounds or azeotropes withinv a narrow boiling range. An'example is an aqueous mixture of ethyl alcohol (anhydrous, B. P. '78.5 C.; aqueous azeotrope, B. P, 78.1-C.) with isopropylalcohol (anhydrous, B. P. 82.3 C.; aqueous azeotrope, B. P. 80.4 0.). Otherexamples include narrowboiling range mixtures containing such alcohols and neutral `oxygenated organic compounds of other classessuch as lcetones, aldehydes, `ether-s,

esters, ketels, andacetals, whenjsuch components or'their azeotropesfboil within anarrow range.

Some ofthe/aboVe-described mixtures are obtained by-an;olefinhyd-rationreaction, e.A g., when a mixture of ethylene and Vpro-pylene is absorbed in sulfuric acid, diluted, hydrolyzed, and a resulting aqueous'alcohol mixture is stripped out. Another important source of such mixtures is the Fischer .synthesis hydrogenation of carbonmonoxide, especially when the aqueous layer product formed contains not only lower primary and secondary alcohols but also various ketones, kaldehydes, ethers, acetals, esters, carboxylic acids,.and certain tertiary alcohols. Still another vsourceis in the products of hydrocarbon oxidation where both oil and water layers are obtained, both containing oxygenated organic compounds.

It is quite feasible to obtain the narrow-boiling range mixtures containing two or more close boiling oxygenated organic compounds from .crude aqueous mixtures by conventional distillations. Narrow-boiling range mixtures which may be obtained by the ordinary distillation processesfrom aqueous solution are as follows:

TABLE I Narrow-boiling range alcohol 'mixtures The narrow-boiling range mixture may be 'a binary or tertiary mixture, as in the groups shown, but, generally, the crude mixtures contain additional oxygenated organic compounds, which do Vnot interfere with the basic operationof this invention in isolating the principal alcohol Ycomponents of the mixtures. It is to be noted that the aqueous azeotropes of the alcohols in such mixtures have boiling points which differby less than ve centigrade degrees.

A typical crude ethanolcutrobtained :from la. water layer of aFischer synthesis processcontains the following:

TABLE II Ethanol cut Aqueous Anhydrous Com ounds o YAzeotropes p Bft" Cj nrt., o.

.'n-Butyraldehyde 75. 7 65.

Eth Acetate 77. l 70.4 Ethyl Alcohol 78. 5 78. 1 Methyl Ethyl'Keton 79. 6, .73. 6 Isopropyl Alcohol. 82.3 '80. 4 t-Butyl Alcohol 82. 8 "79. 9 Methyl Propyl Ketone `101. 7 82.9 ce l 103.2 82.6 Water 100.0

In such crude ethanol cuts, the kinds and quantities of the components are greatly varied but the major components are generally ethyl alcohol, isopropyl alcohol, and methylethyl ketone. Repeated fractional distillations of the ethanol cuts were found to be of no avail for obtaining isolation of pure ethyl alcohol or pure isopropyl alcohol. The difficulties encountered can be appreciated by reference to Table LI which shows the overlapping of the boiling points and by considering that these compounds form additional azeotropes with one another. With the present invention it was found possible to effect the critical separations necessary in recovering the pure alcohols freed of the other substances normally boiling in the same narrow range, even though the contaminating substances have relatively lower and higher boiling points.

To obtain the desired separation of purified organic components from any narrow-boiling range mixture like those mentioned with benefits of the present invention, the mixture may be subjected to a continuous fractional distillation in a column of practical size, including a rectification zone and a stripping Zone for countercurrent vapor-liquid contact under reboiling and reuxing conditions. A sufficiently large quantity of Water is introduced at an upper part of a rectification zone for effectively modifying the relative volatilities of the organic compounds to be sepal rated, and distillation of a larger part of one component than of another from the internal reflux is effected.

The separation can be maintained under steady state conditions to obtain product streams of desired purities and constant compositions while supplying the large quantity of water feed to the upper part of the rectification zone. The temperature of this water feed is preferably close to the temperature of liquid on the water feed plate, although it may be lowered to partially condense vapors ascending to the water feed plate.

Since the efilcient operation is essentially continuous, the water has to be added continuously near the top of a fractionating column while the narrow-boiling range mixture of oxygenated organic compounds to be separated is fed continuously into the column at a lower point, and while sufficient heat is provided to afford distillation throughout the column.

The feed stream of the organic compounds is preferably introduced into a fractionating column between an upper rectification section i and a lower stripping section at a point where the ratio of the main organic compounds to be separated in the feed is similar to the ratio of these compounds in the internal reflux descend- Ving through the column.

The feed stream is preferably preheated to a temperature close to that of the internal liquid reflux under equilibrium boiling conditions at the point of introduction. The preheated feed stream may be liquid, partially vaporized, or completely vaporized when introduced into the fractionating column.

Vapors of the organic compounds introduced as a feed stream at the bottom part of a rectification zone in a fractionating column pass up through the rectification zone in contact with descending internal liquid reflux under equilibrium reboiling and refluxing conditions.

Due to the fact that water considerably in excess of the amount of Water distilled is introduced to mix with condensate near the top of the rectification zone, the water concentration in the internal liquid reflux at the tcp of the rectification zone is higher than the water concentration in vapors passing up through the zone. Contrasted therewith, in normal rectification of alcohols from an aqueous feed, the water concentration diminishes rapidly toward the limiting water concentration of the aqueous azeotropes in the vapors ascending the fractionating columnV The quantity of water required to be introduced continuously at the upper part of the rectification Zone for accomplishing the desired separation of the close-boiling alcohols is considerably greater than the quantity of condensate with which it becomes homogeneously mixed in order to make the water concentration of the internal reflux substantially above a critical minimum in the range of 65-85 mol per cent. With adequate water concentration in the internal reflux for effecting the separation, the organic component to be isolated in the aqueous bottoms is dissolved in the aqueous internal reflux that reaches the bottom part of the rectification zone and finally the bottom of the stripping Zone.

The minimum water concentration of the internal reflux for obtaining the separation depends on the particular organic compounds to be separated. In a limiting case of isolating ethyl alcohol from isopropyl alcohol, essentially no separation is effected if the internal reflux contains less than 65 mol per cent water; and for obtaining satisfactory results on a practical scale, more than mol per cent water, preferably 90-99 mol per cent water, is required in the internal liquid reflux. As the water dilution of the internal reflux becomes infinite, the selectivity of separation is increased but the efficiency is excessively lowered on account of the relatively small quantities of the organic compounds involved.

Under steady state conditions in a fractional distillation zone, the internal reflux having adequate water concentration for accomplishing the separation of the close-boiling alcohols tends to have a nearly constant water concentration in a homogeneous liquid phase at each plate and the high water concentration is approximately uniform in the internal reflux throughout the fractionation zone sections. This internal reflux in flowing from the top to the bottom becomes richer in the ethyl alcohol while the other organic components of the feed becomes distilled overhead.

The overhead vapors from the rectification zone are enriched in one or more of the organic components rendered relatively more volatile by the high water concentration in the liquid reflux While the remaining portion of the organic material introduced with the feed is dissolved in the internal reflux. Thus, if isopropyl alcohol substantially free of ethyl alcohol is withdrawn as overhead product with accompanying water Vapor from the rectification Zone, ethyl alcohol can be removed in the residual reflux at the same rate that the ethyl alcohol is introduced with the feed to the rectification zone.

The functioning of the stripping Zone may be described as follows:

The dilute aqueous solution of the close-boiling alcohols to be separated, as in the liquid reflux from the bottom of the rectification zone, flows downwardly through the stripping zone in countercurrent contact with ascending vapors evolved from the solution under reboiling conditions. A

5. suiciently high concentration of water is maintained in the liquid flowing. down through the stripping zone, as in the rectification zone, to make the liquid progressively richer in ethyl alcohol While the isopropyl alcohol is stripped from the liquid. Under equilibrium reboiling andrefluxing conditions in the stripping Zone, the isopropyl alcohol may be removed as vapor overhead from the stripping zone at the same rate that this alcohol enters the stripping zone as part of the liquid feed to this zone, and a dilute aqueous solution of the ethyl alcohol freed of isopropyl alcohol may be withdrawn from a bottom part of the stripping zone.

A variety of mechanical arrangements may be used for conducting the process, and. suitable arrangements are shown diagrammatically in the drawings.

Figure l of the drawing illustrates av flow plan of a unit for obtaining separation of' two products, e. g., a puried alcohol bottoms product and an overhead product, which are concentrated as such.

Figure 2 illustrates a flow plan on an` expanded unit for accomplishing further separations of overhead products.

Referring to Figure l, I represents a fractional distillation column in the interior of which is provided means for obtaining eiiicient countercurrent liquid-vapor phase contacting, e. g.,` such conventional means as bubble plates, or packing. The crude feed of mixed organic compounds, including the Water-miscible alcohols to be subjected to separation, is charged to column I from an inlet 2 at a point at the bottom of the rectification zone. The Water to be added in large quantity as a refluxing medium enters the column I from line 3. Overhead vapors are withdrawn from the top of column I through line 4 by which they are passed through condenser 5 to a receiver 6. A portion of condensate collected in receiver 6 is returned to the top part of column I as external reflux through line l. The remaining portion of distillate collected in receiver 6 is. withdrawn through line 8.

Bottoms liquid collected at the lower part of column I is passed by line Sinto a reboiler Ill for heating by indirect or direct heat exchange with a heating medium such asl live steam. A portion of the bottoms liquid heated andA partially vaporized in the reboiler I0 is recycled by line II to the lower part of column I. The remaining portion of the bottoms liquid is withdrawn through line I2.

If sufficient fractionation is provided in a secondary rectifying zone above the Water feed' inlet-of line 3 to column I, the distillate collected in receiver 6 may have a composition of a water azeotrope, or approaching thereto, for the higher molecular weight or selectively volatilized' organic compound distilled overhead.

With an excess of water present in the distillate with respect to the azeotropic composition, the distillate requires further processing for separation of excess water, as, for example, by azeotropic distillation or dehydration, such as may be carried out by passing the distillate through line 3 into a Water stripping or concentrating column I3. Column I3 is equipped in a conventional manner' with rectifying means, overheadl line Ill, cooling condenser I5, a distillate receiver I6, and an external reflux return liney Il.

In operating the column I3 for' azeotropic distillation; the; aqueous azeotrope distillate is withdrawn from the system through line. I 8; Stripped Water bottoms. collected in. the; lower part of column I.3 may be: partly recycled. byv lines I9, 20and 2 I through'a heat exchange'reboiler 22 for transferring' heat` directly or through a heat transfer surface, and aremaining portion of the stripped.. Water bottoms from column I3 may be passed through line 23 into the water feed line 3 of column I', thus: conserving on heat energy and makeup Water.

The dilute aqueous solution ofthe isolated organic' compound collected in the bottom of column I' and withdrawn through line I2 may be further processed by azeotropic distillation in a Water' stripping or concentrating column" 25'. Column 25 is also equipped in a conventional manner for fractional distillation with liquidvapor phase contacting means for fractionation, an overhead' lineA 26 for passing overhead vapors to cooling condenser 2l, a receiver 28 for distillate from the condenser', a reflux line 29 for returning a portion of distillate to the upper part of column 25", and distillate withdrawal line 3U. Strippedv Water bottoms collected at the lower part of column 25 are recycled'- by lines 3|, 32, and 33 through a heat exchange reboiler 34,. and. the remaining portion of the bottoms may be passed by line 35y into the water feed line 3 for column I.

When water in excess of the amounts of Water in overhead products I8 and 3l) and that used for recycling isintroduced into the system, e. g.,

with the feed through line 2, into any of the reboilers IB, 22,. andy 34 for direct heating by open steam, or any combination of these, this excess is withdrawn continuously in controlled amounts through line 36 from the bottom of column 25. Fresh makeup Water may be added suitably from line 3l, anda-heat exchange unit 38 may be placed in line 3 for controlling the feed water temperature.

Referring to Figure 2 of the drawing, an initial narrowv boiling range mixture, e. g., an aqueous mixture' containing ethyl alcohol andl isopropyl alcohol whichv are to be isolatedr from each other and' from other components, such as' tertiary butyl alcohol and methyl ethyl ketone, is fed to a rst fractional distillation column 4I.

In column 4I: the ethyl' alcohol is separa-ted from the other organic components: of the mixture.. successively,4 the next higher molecular weight'alcohol isopropyl alcohol, is similarly isolated from the otherl organic components of thev distillate product from column. 4I in column 42, then any tertiary butylk alcohol present in the distillatev product from; column l2 may be separated from methylv ethyl ketone and other organic components inA column" 43. Additional columns may be employed to concentrate or separate methyl ethyl ketone and other organic components in thev distillate products of columns 42 and 43.

Column 4I, like column' I. in Figure 1, is equipped with anY organic mixture feed inlet 44 between itsupper rectification section and its lower strippingsectiom with an upper water inlet 15, overhead vapor line' 4'6-, cooling condenser 4l, external reflux line 48,. and means te that supplies heat to thev bottoms for-reboiling. In this instance, anopen steam linev is indicated to supply heatl directly for. reboiling.` the bottoms, althoughzindirect: heatexchange could be' used.

The bottoms from the separation column 4| is withdrawn by line to a concentration column 5 i, which is equipped and operates like column 25 in Figure 1 to concentrate the isolated alcohol in dilute aqueous solution by azeotropic distillation.

Residual Water is recycled by line 52 for further use as refluxing medium. Surplus water bottoms of column 5| is withdrawn through line 53. The overhead product from column 5| is passed by line 54 to cooling condenser 55, a portion of the condensed overhead product is reiluxed to column 5f by line 56, and another portion of the condensate is Withdrawn through line 5l as finished distillate product. A finished distillate product thus obtained is about 96 volume per cent ethyl alcohol, about 4 volume per cent water, and less than 0.1 per cent of other organic compounds as impurities.

Column 42 in Figure 2 receives as its feed the crude isopropyl alcohol distillate separated from ethyl alcohol in column 4|. This crude isopropyl alcohol distillate is passed from condenser 4l through line 58 into column 42 between its upper rectification section and its lower stripping section. The crude isopropyl distillate may generally contain methyl ethyl ketone and tertiary butyl alcohol with a substantial proportion of water and several other organic components which pass overhead from column 4|.

Column 42 is equipped like column 4i with an overhead vapor line 59, a cooling condenser 55, an external reflux line 5i, a water feed line 52, and a bottoms heating line 63. Purified isopropyl alcohol in dilute aqueous solution is separated as bottoms in column 42 from the other organic components of the feed in substantially the same manner as the ethyl alcohol is separated in column 4| and preferably by maintaining a water concentration of 90-99 mol per cent in the internal reflux that descends through column 42.

Withdrawal line S4 passes the puried isopropyl alcohol in dilute aqueous solution to concentrating column 55 which operates like column 5i to concentrate the isolated isopropyl alcohol by azeotropic distillation. Residual water from column 65 is recycled by line 52. Surplus water bottoms of column 55 may be withdrawn through line 56. The overhead product from column 55 is passed by line 61 to cooling condenser 6B from which a portion of the distillate is refluxed by line 59. The remaining proportion of the distillate is withdrawn from the condenser by line 'l0 as finished product. A concentrated isopropyl alcohol thus obtained is substantially free of methyl ethyl ketone, tertiary butyl alcohol, and such other organic components initially present in the feed mixture to column 42.

Column 43 receives as its feed an aqueous distillate product withdrawn from condenser Si! by line 'f i. This distillate may contain tertiary butyl alcohol, methyl ethyl ketone, and other organic components in aqueous solution.

Column 43 is equipped and is operated like the preceding columns 4| and 42 for separating tertiary butyl alcohol from methyl ethyl ketone and the other organic components present in the feed delivered by line 1|. The feed enters column 43 between its upper rectification section and lower stripping section. Water is charged into the upper part of the column by line l2. Overhead vapors are passed by line 'i3 to cooling condenser 74, from which a portion of overhead distillate is returned to column 43 by reflux line '15. Heat for reboiling is supplied by steam from line 1 5.

By maintaining a suitably high water concentration, preferably of 90-99 mol per cent, in the internal reflux descending through column 43, purified tertiary butyl alcohol dissolved in the water bottoms is isolated from the other organic compounds of the feed. The bottoms of column 43 are withdrawn by line 'Il to the concentrating column 'I8 for azeotropic distillation of the alcohol. Column 18 is equipped with overhead vapor line 79, cooling condenser 80, external reux line 8|, and product withdrawal line 82. Water bottoms are recycled from column I8 by line 12 for further use as reiuxing medium in column 43. Surplus water bottoms of column 13 are withdrawn through line 89.

The concentrating columns 5|, 65, and 18 are provided with lines 53, 34, and which supply steam for reboiling. The lines 52, E2, and 'f2 used for recycling the water bottoms are provided with pumps 86, 87, and 83, respectively. Other equipment of conventional types useful in fractional distillation apparatus may be employed even though omitted from the drawing for the sake of simplicity.

By operating the overhead condensers of each separating column in a series e. g. the condensers 47, 60, and 'i4 shown in Figure 2, the overhead products forwarded as from column 4| to column 42, and from column 42 to column 43 may be maintained partly or entirely in vapor phase to thus reduce heating requirements.

Data obtained in the application of the process for the removal of isopropyl alcohol from ethyl alcohol in a narrow boiling aqueous mixture of these alcohols clearly demonstrate that the admixture of sufficiently large amounts of water with the reflux in the fractional distillation zone makes the isopropyl alcohol much more volatile than the ethyl alcohol and is able to reduce the isopropyl alcohol in the bottoms to a very low concentration, even to the extent that the iso propyl alcohol is not detectible in the residual ethyl alcohol solution.

Representative data are given in the following tables:

TABLE III Relative volatilz'tz'es of isopropyi alcohol (IpOH) to ethyl alcohol (EtOH) in presense of ecess water [92 mol per cent H2O in liquid pliase alcohol analyses expressed on Water-free basis] Relative lqui- Mol Mol Mol Volatility i librium Phase Per Cent Per Cent Per Cent IpOH to Mixture H2O Et IpOH EtOH (Alpha) 10. 7 89. 3 2 i5. 6 si. 4 i. 44

50. 6 49. 4 4 {Vapo r. 54 o3. Ai 31.6 1.55

Liquid.. 92 77.0 23. (l

5G Sl. 5 18. 5 l. G2 92 S7. 6 12. 4 6 56 93. l G. 9 l` 72 Liquid 92 fl 1 The relative volatility is the volatility of one component divided by that of the other, the volatility of cach component being propor tional to itspartial pressure divided by its mol fraction in tbe liquid phase. It is also defined by the equation .\l1ilia=(y,/y;)/(x1/r2) where y refers to the vapor phase mol fractions of tbc components to be separated and I refers to the liquid phase mol fractions of the components to be separated, subscript one designates the moro volatile component and subscript two the less volatile component.

assises [97 mol per cent H2O in liquid phase alcohol analyses expressed on water-[rec basis] Equilib- Mol Mol Mol Alpha brium Phase Per Cont Per Cent Per Cent IpOH to Mixture H2O EtOH IpOH EtOH l [Vapor 63 7. 9 92.1 l. 54

lLiquid 96 11.6 83.4 2 {Vapor 14.5 85. 5, 1. 54

Liquid -96 20. 7 79. 3 3 Vap01' 64 40. 5 59:5 1:55

Lquid .97 51.2 48. 8, 4 {Vapor 69 .80. 6 19. 4 1. 77

Liquid 97 88. 0 12.` 0 5 {vermi-..- 91. 7 8.3; A1.80

Liquid 95.2y 4.8 6 {Vapor 71 '93. 3- 6:7 41.84..

Liquid 97 9,6. 3 3. 7

The `data in ATables lll and .IV indicate Ayiery clearly that-the large propcrtion-oiwater present in the liquid phase with ythe mixed .alcohols increases therelativevolatilityof the higher molecula-r weightisopropyl alcohol with respect .to the lowei` molecular weightethyl alcohol, regardless of which alcohol is in preponderance initially.

To conrm that complete .separation .of iso.- propyl alcohol `-from ethyl-.alcohol is practicablein a continuous fractional distillationcolumn .operated iii-accordance with principles outlined, .a column with rectiiyingandstripping sections, .an intermediate feed section, a .top water feed section, and condenser was used. Aqueous .blends of isopropyl alcoholand ethyl .alcohol andblends with other-close-boiling oxygen-containing compounds were subjected .to .continuous Ifractional distillationin the `column supplied near the .top with Water.

4For the purpose oi illustration but without limiting the invention, the .following examples are given:

An alcohol Volend of 4.6 weight per .cent isopropyl alcohol in ethyl alcohol with -5Weight per cent'water-was used as the feed to the continuous fractional distillation column having two '30- plate sections. With the alcohol mixture feed rate maintained at l52yolu1nes perhour to apoint between-the two sections, water preheated'to 590 C. was fed into the top ci the column Vat aflow rate of lil'volumes per '-hour. The-external reflux ratio was somewhathigherthan 30:1. After equilibrium conditions wcreobtained Ain the column, the isopropyl alcohol concentration in the aqueous ethyl alcohol lcottoms was maintained below 0.1 weight per cent alcohol basis, which is well below the specification requirements on high-quality vethyl alcohol.

EXAMPLE '2 Ina run carried out inthe same manner asin Example 1, a small amountof methyl ethyl ketone was included in the feed in additon to isopropyl alcohol and ethyl alcohol. [No detecti'ole concentration of methyl ethyl ketone or isopropyl alcohol was found in .the aqueous .ethyl alcohol bottoms after equilibrium conditions were reached, .thus indicating that .the vketone actually facilitated the isolation of ethyl alcohol.

ln another run carriedout in the same .manner as lExample -1, an ethanol cut from a Fischer synthesis water layer productcontaining componentsof the .types lindicated in Table Ill `was fractionally.distillediwith 98 mol-percent water in the liquid iphase. 'After lthe column-was Irun ycontinuously Yfor a period to `.reach steady state conditions, :no appreciable amount of isopropyl yalcohol vwasxietectilole `in .the aqueous ethyl alcohol :bottoms jwithdrawn `*from the `f-ractionating column. @ne-half vper lcent methanol was ldetected in thealcoholcontentof the bottoms `4but this wasireduced -to `less than-0.01 weightper'fcent on an alcohol fbasis by :topping the ethyl alcohol product concentrated from vlthe bottoms The A.overhead .distillate 4,product of the fractionatingfcolumnfhas a pungent'odor. lAn-anal-ysis on a .composite :of overhead samples :showed thegpresenceof 1.4% acetal, `3.2% laldehyde, and- 8-.4'51% methyl ethyl fketone with-the distilled iso'- propyl .-alcohol. :No l'carbonyl compounds were detectible in the ethylalcoholfbottoms. -Thusfit was clearly demonstrated that -it was possible to produce specification pure 'ethyl yalcohol yfrom crude water solution products -of a Fischer synthesis.

LEXAMPLEA 'lnpurifying the .iSQDrQpylalcQhol .as contained inthe overhead products of the ethyl alcoholselparation column, the lclose-Jooiling impurities are Similarly rendered more volatile and .distilled overhead.

Analytical distillationvdata ona crudeisopropyl alcoholzfeed to aractional.dstillationcolumn .in which the reiluX ,132Mo Wasz1and a concentra.- tlOn ,of 59.2 molner cent water was malntanedln theinternalreflux indicateggl nearly complete.separation. With as much as `66.7 vol. .per cent methylethyllietone on a .Water-.freebasisin the feed. .the bottoms .contained over .'98 .weight per Cent isopropyl .alQhQln a water-.fnac basis ...and the overhead wasifree ojsopropylalcohol.

Also, withtertiary butyl alcohol present in iso.- nropyl alcohol .feedpngder .equilibrium .conditions with [9,1 .mol .per `.Cent Waterin the liquidphase,

it was found .thai/.the `relative .volatility o f .jf-,butyl alcohol ,over isopropyl alcohol ,is about .1.4.

Analytical data on .the separation of methyl ethyl ketone from vt-butyl alcohol showed 'that' with 9 1 mol per cent water maintained `in ,the liquid phase'the Arelative -Volatility of the v`ketone over this alcoholis ofthe order of 11:7. Thus, t-v

butyl alcohol ips-readily separated from methyl ethyl ketone Awith no .ethyl alcohol or isopropyl alcohol present inthe feed tothe column used for separating the t-"butyl alcohol.

By simple distillation of aqueous 'phase lproducts from Fischer syntheses, six distinct distillates boiling y'loelow I'100 C. were obtained. A representative fractionation of such jproduct is shown in the following table:

Cuts `from vvfractional -distillation of aqueous 'layer Vapor ein Cut Major Components lAcetaldehyde, Methanol. iPropionaldehyde; Ester. .-ACtone..AcemLMethanoL Ethanol, IsopropanoLKetones. n-Propanol, isoand'secButanols. zweier..kleben-alcohols.

Y A typical ethanol cut boiling from about 69 to 82 C.- thus obtained comprised about 5 weight per cent water, 72.5 weight per cent ethyl alcohol, 5.3 weight per cent isopropyl alcohol, 11.3 weight per cent ketones, 0.2 weight per cent t-butyl alcohol and other neutral organic substances identified as in Table 1I. By subjecting this cut to fractional distillation with 90-99 mol per cent water in the internal liquid reflux as described in Example 3, ethyl alcohol recovered in the bottoms was over 99.5% pure on an anhydrous basis.

By further distillation of the overhead distillate containing isopropyl alcohol with use of high water concentrations (90-99 mol per cent) in the internal reflux as described in Example 4, it was found possible to obtain a similar isolation of isopropyl alcohol from the ketones and the other organic components, or if desired, to separate an aqueous bottoms in which only isopropyl alcohol and t-butyl alcohol remained.

The process of this invention can alsq be used for making less exact separations of the alcohols, than illustrated in the above examples, in which simply the relative proportions of the alcohols are altered by distillation through a series of stages in which a high water content is maintained in the refluxing liquid phase, the number and efficiency of the stages determining the degree of the separation obtained. This process can thus be used for the complete or partial separation, as desired, of any mixture of aqueous azeotrope-forming, water-miscible alcohols, especially those of 2 to 4 carbon atoms per molecule, with or without the presence of other neutral oxygenated organic compounds, even when these or their azeotropes with water have boiling points close to or within the range of the alcoholwater azeotropes in the mixture being separated. If the initial alcohol mixture contains methyl alcohol, it can be separated from the remaining alcohols either by initial fractionation of the mixture or by subsequent distillation of the distillate and residual fractions obtained in the present process. This is possible because of the relatively low boiling point (64.7 C.) of methyl alcohol and the fact that it does not form an azeotrope with water.

The use of salt dissolved in the water refluxing medium makes it possible to further increase the relative volatility spread of the close-boiling alcohols. Salt solutions offer the further advantage of satisfactorily lowering the concentration of 'water in the overhead product, the salt being selected with care to avoid undesired effects of corrosion and catalyzing the reactions of organic compounds.

lAlthough generally the fractional distillation is to be carried out at about atmospheric pressure, it may also be carried out at pressures below or above atmospheric pressure.

As already mentioned, the overhead and bottoms products from the fractional distillation in the presence of excess Water can be further processed to remove water by physical and/or chemical dehydrating, e. g., azeotropic distillation treatment, with chemical drying agents, or the like.

We claim:

l. The method of separating water-miscible alcohols which form close-boiling aqueous azeotropes difficult to separate by ordinary fractional distillation in having boiling points which differ by less than 5 C., one of said alcohols having a higher molecular weight than the other, which comprises continuously introducing a feed mixliquid reflux from a bottom part ture of said alcohols to a fractional distillation zone wherein vapors of said alcohols with water vapor ascend countercurrently to liquid reflux of the alcohols dissolved in a sufficiently higher proportion of water to effect vaporization of a larger part of the higher molecular weight alcohol than of the other alcohol in said reflux, continuously introducing water at an upper part of the fractional distillation zone to maintain at least G5 mol per cent water in said liquid reux, continuously removing vapor of the higher molecular weight alcohol with water vapor overhead from the fractional distillation Zone, and removing a dilute aqueous solution of the other alcohol as bottoms.

2. The process of separating water-miscible alcohols from a mixture with other oxygenated organic compounds, particularly of the class consisting of ketones, aldehydes, esters. ethers, ketals, and acetals, that form close-boiling azeotropes diicult to separate by ordinary fractional distillation in having boiling points which differ by less than 5 C., which comprises continuously introducing a feed mixture of the alcohols with said other compounds to a rectification Zone wherein vapors of the feed mixture ascend countercurrently in contact with liquid reflux of condensate from said vapors with increased water content, increasing the water content of condensate at an upper part of the rectification zone by introducing water continuously thereto in excess of the amount of water being distilled from said zone so that the liquid reflux contains at least 65 mol per cent water, flowing internal of the rectification zone countercurrently in Contact with vapors boiled from the internal liquid reflux as it iiows down to a bottom part of the stripping zone, and withdrawing from the bottom part of the stripping zone adilute aqueous solution of one of said alcohols freed of other organic components of the feed mixture.

3. In a process of separating ethyl alcohol from isopropyl alcohol, the steps which comprise continuously passing a dilute aqueous solution of ethyl alcohol containing isopropyl alcohol down through a stripping Zone so that a liquid portion of said solution flows countercurrently in contact with vapors evolved therefrom under constant refiuxing and reboiling conditions, maintaining a sufficiently high concentration of at least 65 mol per cent water in the resulting internal reflux to effect vaporization of a larger part of the isopropyl alcohol than of the ethyl alcohol in said reflux, continuously withdrawing vapors of water mixed with vapors of the alcohols overhead from the stripping zone, the isopropyl alcohol being thus withdrawn as vapor at the same rate that the isopropyl alcohol dissolved in said aqueous solution enters the stripping Zone, and withdrawing from a bottom part of the stripping zone a dilute aqueous solution of ethyl alcohol substantially free of isopropyl alcohol.

4.1n a process of separating alcohols mu'- tually niiscible with Water and which form closeboiling aqueous azeotropes difficult to separate by ordinary fractional distillation in having boiling points which differ by less than 5 C., the steps which comprise continuously passing into an upper part of a stripping zone a dilute aqueous solution of the alcohols so that a liquid portion of said solution flows downwardly through the stripping zone in countercurrent contact with vapors evolved from the solution under constant accuses refluxing and reboiling conditions, maintaining a sufciently high concentration of at least 65 mol per cent Water in the resulting internal reflux to effect vaporization of a larger part of one of said alcohols than of another of said alcohols having a lower molecular weight, the alcohol thus rendered more volatile being withdrawn overhead as vapor at the same rate that this alcohol dissolved in the aqueous solution enters the stripping zone, and withdrawing from a bottom part of the stripping zone a dilute aqueous solution of said alcohol having a lower molecular weight substantially free of the alcohol rendered more volatile.

5. In a process as described in claim 4, the

concentration of water maintained in the internal reiiux being of the order of 90 to 99 mol per cent.

6. In a process as described in claim 4, the alcohols mutually miscible with water including isopropyl alcohol and tertiary butyl alcohol.

7. In a process of separating alcohols mutually miscible with water and which form close-boiling aqueous azeotropes difficult to separate by ordinary fractional distillation in having boiling points which differ by less than C., the steps which comprise continuously passing vapors of said alcohols up through a rectification zone wherein the alcohol vapors mixed with water vapor ascend in contact with a countercurrent internal reflux containing condensate from said vapors with increased water content, giving the internal reflux its increased water content of at least 65 mol per cent thereof by continuously introducing liquid water into condensate at an upper part of the rectication zone, said liquid water being heated to a temperature adequately high to keep the condensate mixed therewith in a substantially smaller proportion steadily boiling,`continuously removing overhead from said rectification zone the vapor of one of said alcohols volatilized to a greater extent than another of said alcohols having a lower molecular weight by the increased water content of the internal reux, and continuously withdrawing from a d bottom part of said rectification Zone residual internal reflux which is a dilute aqueous solution of the alcohols.

8. In a process as described in claim 7, the water content of the internal reflux being of the order of 90 to 99 mol per cent.

9. In a process as described in claim '7, isopropyl alcohol being one of the alcohols Volatilized to a greater extent, and ethyl alcohol being the other alcohol having a lower molecular weight.

10. In a process as described in claim 7, tertiary butyl alcohol being one of the alcohols volatilized to a greater extent.

.fil

11. In a process of separating ethyl, isopropyl,

and tertiary butyl alcohols from a mixture of these alcohols, the steps which comprise fractionally distilling the mixture in a first fractional distillation zone wherein internal liquid reux has a water concentration of to 99 mol per cent, withdrawing as bottoms of rst fractional distillation zone a dilute aqueous solution of the ethyl alcohol, removing as overhead distillate from said first fractional distillation zone an aqueous solution of the isopropyl and tertiary butyl alcohols, fractionally distilling said distillate in a second fractional distillation zone wherein internal liquid reflux has a water concentration of 90 to 99 mol per cent, withdrawing as bottoms of said second fractional distillation zone a dilute aqueous solution of the isopropyl alcohl, and distilling tertiary butyl alcohol overhead from said second fractionation Zone.

12-. In a process of separating isopropyl alcohol and methyl ethyl ketone from ethyl alcohol, the steps which comprise fractionally distilling isopropyl alcohol with methylethyl ketone from a mixture containing ethyl alcohol in a fractional distiliation zone wherein internal liquid reflux is given a Water concentration of 90 to 99 mol per cent, and withdrawing as bottoms a dilute aque-- ous solution of ethyl alcohol substantially freed of isopropyl alcohol from the fractional distillation zone.

13. A process of isolating an alcohol from aqueous solution products synthesized by reaction of carbon monoxide with hydrogen, the steps which comprise fractionally distilling from said products a narrow boiling out containing neutral oxygenated organic compounds including alcohol components which form aqueous azeotropes that diifer in boiling points by less than 5 C., selectively distilling organic components of said cut in a fractional distillation zone by maintaining a water concentration of 90 to 99 mol per cent in liquid internal reflux Within the zone to where the internal reflux contains one of the alcohol components to be isolated and is stripped of the other organic compounds to be separated therefrom.

EDWIN R. GILLILAND. CHARLES E. MORRELL. CARL S. CARLSON. NAT C. ROBERTSON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,290,442 Metzl July 21, 1942 2,348,683 McKenna May 9, 1944 2,386,058 Patterson et al. Oct. 2, 1945 2,392,534 Von Keussler Jan. 8, 1946 FOREIGN PATENTS Number Country Date 598,346 Great Britain Feb. 17, 1948 

1. THE METHOD OF SEPARATING WATER-MISCIBLE ALCOHOLS WHICH FORM CLOSE-BOILING AQUEOUS AZEOTROPES DIFFICULT TO SEPARATE BY ORDINARY FRACTIONAL DISTILLATION IN HAVING BOILING POINTS WHICH DIFFER BY LESS THAN 5* C., ONE OF SAID ALCOHOLS HAVING A HIGHER MOLECULAR WEIGHT THAN THE OTHER, WHICH COMPRISES CONTINUOUSLY INTRODUCING A FEED MIXTURE OF SAID ALCOHOLS TO A FRACTIONAL DISTILLATION ZONE WHEREIN VAPORS OF SAID ALCOHOLS WITH WATER VAPOR ASCEND COUNTERCURRENTLY TO LIQUID REFLUX OF THE ALCOHOLS DISSOLVED IN SUFICIENTLY HIGHER PROPORTION OF WATER TO EFFECT VAPORIZATION OF A LARGER PART OF THE HIGHER MOLECULAR WEIGHT ALCOHOL THAN OF THE OTHER ALCOHOL IN SAID REFLUX, CONTINUOUSLY INTRODUCING WATER IN SAID LIQUID REFLUX, THE FRACTIONAL DISTILLATION ZONE TO MAINTAIN AT LEAST 65 MOL PER CENT WATER IN SAID LIQUID REFLUX, CONTINUOUSLY REMOVING VAPOR OF THE HIGHER MOLECULAR WEIGHT ALCOHOL WITH WATER VAPOR OVERHEAD FROM THE FRACTIONAL DISTILLATION ZONE, AND REMOVING A DILUTE AQUEOUS SOLUTION OF THE OTHER ALCOHOL AS BOTTOMS. 